Organ development requires a tightly controlled program of cell proliferation followed by growth arrest and differentiation and, often, programmed cell death. The balance between the number of cell divisions and the extent of subsequent programmed cell death determines the final size of an organ (reviewed by Bryant and Simpson, Quart. Rev. of Biol., 59:387-415 (1984); Raft, Nature, 356:397-400 (1992)). Although much of the cellular machinery that determines the timing of onset and cessation of cell division per se is well understood (reviewed by Hunter and Pines, Cell, 79:573-582 (1994); Morgan, Nature, 374:131-134 (1995); Weinberg, Cell, 81:323-330 (1995)), little is known about the signals that cause discrete groups of cells and organs to terminate growth at the appropriate cell number and size. A better understanding of the signals involved provides possible targets for manipulating the cellular machinery resulting in therapeutic benefits for a number of conditions.